Reaction of Acyclic Hydrocarbons Towards t-Butoxy Radicals. A Study of Hydrogen Atom Abstraction by Using the Radical Trapping Technique

The reaction of 3-methylpentane and 2,4-dimethylpentane toward t-butoxy radicals has been investigated, in neat and benzene solutions, by using the radical trapping technique. Abstraction occurs principally from the tertiary and secondary C-H reaction sites of 3-methylpentane and the tertiary position of 2,4-dimethylpentane. The tertiary and in particular secondary C-H reaction sites of 2,4-dimethylpentane are shown to be considerably less susceptible towards t-butoxy radical facilitated abstraction compared with the equivalent reaction sites of 3-methylpentane. As a result, the latter is three times as reactive as 2,4-dimethylpentane as a neat hydrocarbon solution and seven times as reactive in a diluted mixture of benzene. Diferences in selectivity and rate of hydrogen abstraction, between the substrates, are interpreted in terms of non-bonding interactions retarding t-butoxy radicals from approaching sterically demanding C-H reaction sites. The selectivity from 3-methylpentane is solvent-insensitive whereas abstraction from 2,4-dimethylpentane is modified in benzene. Further, the rate of hydrogen abstraction, from either substrate, to t-butoxy radical β-scission is considerably smaller in benzene. Both observations are interpreted in terms of t-butoxy radical solvation by the aromatic solvent.